Many types of articles are provided with coatings by high temperature, high velocity coating processes such as detonation gun plating, jet-plating and arc torch methods. After these coated parts have been in service sufficiently long to wear to a point where replacement is necessary it is desirable to have a method for quickly and economically removing the worn coating so that the base part can be coated again and placed back in service. In many instances the base part involves considerable expense in its original manufacture, so that reclaiming such parts can be of great importance. Also, in the course of applying wear resistant coatings it may occasionally be necessary to remove coatings that do not meet specifications, in which case reclaiming the base part is again desirable.
The prior art has devised several methods of removing coatings, such as refractory coatings, including mechanical removal by grinding. On cylindrical parts, for example, the coating may be ground off down to the base metal with removal of a small amount of the base metal below the original dimension to insure complete removal of the old coating and permit reacting. It has been found, however, that such procedure is time consuming, expensive, and not always advisable since grinding away a portion of the base metal to insure complete coating removal prevents the reclaimed base material from conforming to the original dimensions as specified by its user. Also, parts which are not cylindrical often may not be ground. Improper coating of such parts may necessitate their replacement and scrapping of the original part with its attendant expense and time delay.
A known stripping method uses electrolytic solutions such as aqueous sodium hydroxide or sodium carbonate. The coated part is immersed in the bath and connected as the anode of an electrical circuit while the steel tank containing the bath is connected as the cathode. This method has been found satisfactory for removal of some coatings but is not suitable for stripping certain mixed refractory coatings such as tungsten carbide-chromium carbide-nickel and chromium carbide-nickel-chromium. Also, the aforementioned sodium hydroxide or sodium carbonate electrolytic baths do not conveniently remove refractory coatings applied by the detonation plating process using inert gas dilution, as more fully described in U.S. Pat. No. 2,972,550.
U.S. Pat. No. 3,151,049 discloses an effective method for electrolytically stripping a substantially oxide-free, metal-containing refractory coating from a base material in which the coated base part is immersed as an anode in an electrolyte bath container, for example, in a steel tank serving as the cathode. The electrolyte bath for the stripping processes consists essentially of a soluble salt of an hydroxy organic acid, an alkali metal carbonate and the remainder water. Although this electrolyte bath solution is suitable for stripping many types of coatings from different base materials, when the base material is aluminum there is a tendency for the aluminum to be attacked by the alkali metal carbonate such as sodium carbonate. The attack on the aluminum could result in pitting, cracking and/or corrosion of the aluminum.
It is therefore an object of the present invention to provide a method of stripping coatings, such as metal carbide coatings, from all shapes of aluminum base materials in a rapid and economical manner without pitting, cracking and/or corroding of the base material.
It is another object of the present invention to provide a method of and electrolyte bath for stripping coatings from all shapes of aluminum base materials without necessitating removal of any of the aluminum base material.
These and other objects and advantages of the invention will become apparent from the following description.